1. Field of the Invention
The present invention relates to a diffractive optical element and a measurement device using the diffractive optical element.
2. Description of the Related Art
A diffractive optical element that diffracts at least a part of incident light is used in various optical devices, optical apparatuses and the like. As an optical device, for example, an optical three-dimensional measurement device is available that performs three-dimensional measurement by radiating a predetermined light projection pattern to an object to be measured and obtaining an image of the object to be measured to which the predetermined light projection pattern is radiated. In such a three-dimensional measurement device, the diffractive optical element is used for generating the predetermined projection pattern of light.
U.S. Pat. No. 6,101,269 discloses a method in which when three-dimensional measurement is performed, a speckle pattern generated by a diffractive optical element is radiated as the projection pattern of light radiated to the object to be measured. WO2009/093228 discloses a method in which a plurality of diffractive optical elements are used to improve the controllability of the light quantity distribution of the diffracted light.
However, in U.S. Pat. No. 6,101,269, in the speckle pattern, since light spots of high intensity are formed in random positions within the projection surface, roughness occurs in the in-surface distribution of the light spots on the projection surface. For this reason, in a region where no light spot in the speckle pattern is radiated, three-dimensional information cannot be obtained, so that precise three-dimensional measurement cannot be performed. Consequently, the resolution of the three-dimensional measurement device decreases.
When a plurality of diffractive optical elements are used as in WO2009/093228, it is necessary to consider the stray light generated by the diffractive optical elements. That is, when light spots of the diffracted light and light spots of the stray light coincide with each other, the light intensity becomes unstable because of interference of light, so that there are cases where the light intensity enormously decreases. When the light intensity at light spots decreases like this, roughness occurs in the in-surface distribution of the light spots on the projection surface, so that it is impossible to precisely perform three-dimensional measurement and the like.